The present invention relates generally to a power assisted braking system for a vehicle and more particularly to methods and apparatus for providing optimal pedal feel and improved time response for a braking system having, for example, an antilock feature. At the beginning of a brake apply, the fluid displaced out of the master cylinder is used to bring the brake friction material in contact with the brake rotors or drums. This take-up of running clearances is basically lost travel from a pedal feel standpoint. The goal of this invention is to reduce this initial pedal travel and thus to improve pedal feel.
It is desirable to decrease brake response time from a safety viewpoint and desirable to improve the feel of the brake pedal to the vehicle operator from an ergonomic viewpoint. Many known antilock devices operate by cyclically increasing and decreasing a braking force exerted on the wheels so that a slipping wheel having a tendency to lock is permitted to re-accelerate back to a speed corresponding to the speed of the vehicle. This is typically achieved by control valves alternately allowing fluid to flow out of and then into a brake cylinder to first lower and then raise the brake pressure in the brake system. Typically antilock braking systems utilize either a so-called pump-back scheme or a replenish scheme during a reapply or build operational sequence to maintain a desired level of hydraulic fluid in a brake system. In a pump-back scheme, the same hydraulic fluid is re-supplied from a local low pressure accumulator to the brake pad actuators while in a replenish scheme hydraulic fluid comes from a separate source such as either a hydraulic accumulator or a separate pump and motor. Most of such antilock braking systems are further capable of operating in a traction control function. A traction control function is established by detecting conditions where the rotational speed of a first powered wheel substantially exceeds that of a second powered wheel. To provide a power balance in the operation of a vehicle, a braking force is applied to the powered wheel rotating at a higher speed effectively transferring driving torque back to that wheel with better traction. Many antilock systems having such a traction control feature employ a motor and hydraulic pump or pumps along with fluid accumulators which operate somewhat independently of the service braking system.